(a) Controls for Electric Power Apparatus
The general arrangement for control of electric power apparatus in electrical utility power systems is shown in FIG. 1. This apparatus responds to some criteria, including amplitude, of voltage or current signals or both. If two or more input signals are required for a specific application, the phase relation between any two may also be included in the criteria. For purposes of this document, "control" means the timely response to varying measurements in an electrical utility power system, and not to the instantaneous response sometimes required to protect against damage from fault or short-circuit conditions in such systems. Controls may require only one alternating current voltage signal input, such as for use in power factor capacitor controls, which are connected to or removed from electrical power distribution circuits in response to the magnitude of the distribution voltage. Another control for use on a three-phase, four-wire electric power circuit may require three voltage and three current input signals for response to three-phase watts or vars. A simplified version of a control for the same purpose could use one voltage and three current input signals, where it is assumed that the voltages are equal and 120 degrees apart in relative phase angle.
The voltage signal input to the prior art control 17 is from a potential device 14; such as a voltage transformer, capacitor potential device, or a resistive potential device. The current signal inputs, if required, are obtained from sensing device 16, such as a current transformer or line current sensor. Current sensors are air core coils held by an insulator in close proximity to an electric power conductor. The coil picks up a voltage from the magnetic field produced around the conductor by the current flowing in the conductor. Additional potential devices, collectively numbered 13, and current transformers or sensors, collectively numbered 15, are shown to indicate that they may be required depending on the functions of the specific control 17, as described previously.
Control 17 uses one or more output relays, collectively numbered 18, which typically close their output circuit to actuate power system apparatus 19. For example, assume apparatus 19 is the switch mechanism to activate the "raise" or "lower" motor starters of a load tapchanging transformer. In this example, control 17 would have a "raise" output relay and a "lower" output relay 18, which would close accordingly to actuate the "raise" or "lower" motor starters, respectively. In response to the relay conditions, the motor would either raise or lower the tap position to raise or lower the voltage from the transformer. Another example would be "close" and "trip" output relays 18 that would control a power factor capacitor "connect" and "disconnect" switch, which would be used to bring the capacitor on and off line, respectively.